Judges 20:25

20:25 The Benjaminites again attacked them from Gibeah and struck down eighteen thousand sword-wielding Israelite soldiers.

Judges 20:34

20:34 Ten thousand men, well-trained soldiers from all Israel, then made a frontal assault against Gibeah – the battle was fierce. But the Benjaminites did not realize that disaster was at their doorstep.

Judges 20:36

20:36 Then the Benjaminites saw they were defeated.

The Israelites retreated before Benjamin, because they had confidence in the men they had hid in ambush outside Gibeah.